WO-A-2012/038550 describes a cardiac stent-valve and a system for delivering the stent-valve percutaneously. The stent-valve is compressible to a compressed state suitable to be accommodated within two complementary sheaths at the delivery tip of the delivery catheter. In the compressed state, the small size enables the catheter to be introduced percutaneously, and advanced via the femoral artery, to reach the heart. The two sheaths cover different sections of the stent-valve, and are translatable in opposite directions to deploy the sections of the stent-valve in a predetermined sequence. Upon deployment, the stent-valve expands to an operative size.
The task of compressing the stent-valve on to (or ready for) the delivery catheter is complicated because the stent-valve is delicate and vulnerable to damage. Damage may result from over compression, or a nonuniform stress distribution, or buckling, or non-circularity during compression, or from tearing or abrasion of valve component tissue. A deformed or damaged stent-valve may function imperfectly, or have a reduced operational life, or may be difficult or even impossible to implant correctly. The complications are exacerbated in the case of a self-expanding type of stent-valve because a self-expanding stent-valve has a strong restoration force when compressed, and requires application of a large compression force to compress the stent-valve down to its compressed condition. Large forces are difficult to apply to a delicate stent-valve. A self-expanding stent-valve may also have more of a tendency to deform undesirably to a non-circular shape unless the shape is carefully controlled during compression. Further complications arise when the stent-valve is to be compressed for loading on to a delivery catheter having multiple sheaths that close over the stent-valve in different directions. Such sheaths limit the available room and possibilities for compressing the stent-valve.
It remains challenging to provide a technique for compressing a stent-valve, that is relatively easy and intuitive to use, inexpensive to implement, uses apparatus that is not too bulky and can conveniently be sterilized, and also avoids the problems discussed above.